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How should I apply pre-emergence herbicides?

As we are approaching the season for pre-emergence applications, it seems timely to have a look at how best to apply these, both for efficacy and for minimising drift.

Pre-emergency applications are the easiest to balance drift and efficacy, because they actually re-enforce each other. You can’t miss the target when it is as big as the soil, so large droplets are not a problem. Drift-reducing nozzles are therefore a no-brainer.

Pre-em trial conducted by Frontier with us in 2014 showed no effect of nozzle or volume

The first application factor that could affect performance is delivering the right dose to the treated area, and for most of you, that means being applied uniformly over the field. Do do this well, you need to use a spray that will not be diverted by wind gusts or wake from the sprayer itself. Drift-reducing nozzles will reduce the effect of wind and wake, which is why they are good for efficacy in this situation. You also need a low boom, and good boom control – not just of height but also of ‘yaw’, or movement around the direction of travel. When spraying over bare ground, a stable boom height is essential if you are going to keep it low and not risk it hitting the ground.

The other issue relating to uniformity is how good your rate controller is. There will always be something of a time lag between deciding that the flow rate needs to change, and the change being delivered, and if the speed of the sprayer is increasing or decreasing at the same time, it is possible that you are never spraying at exactly the right dose. Some work we did with Househam Sprayers a few years ago showed that you get the most uniform dose delivery when you are spraying at your target speed. This either means not trying to go too fast, so that you reach your target speed quickly, or accelerating/decelerating very rapidly (which could be bad news for other reasons, including boom stability). Househams were able to make changes to their control systems to reduce dose fluctuations at the start and end of tramlines, but not all sprayer manufacturers will have explored this, and it may be impossible to eliminate them completely. Modest sprayer speeds (perhaps around 12 km/h) are probably best in many situations.

The second important factor for all applications is timing If we have a spell of unpredictable weather in late summer/autumn, it could be important to cover the area quickly. If you can’t go as fast as you would like, keeping volume down can be hugely important. This is where we differ from the Syngenta message of 200 L/ha. Published data doesn’t show a benefit of increasing volume, and there is no scientific explanation for why 200 L/ha should be better than 100 L/ha: ‘coverage’ is probably not a relevant factor for soils, and in any case it is clearly not an issue because very large droplets work well; the additional water is far too little to make up for any lack of rainfall – 1 mm rain is equivalent to 10,000 L/ha! Syngenta data may show something different, but we have not been able to see it, so it would not be sensible to change our recommendation. We still believe volumes of around 100 L/ha should work well and deliver good work rates to optimise timing.

The final question is then – what nozzles should you use? You will have to wait for this – I will put some suggestions together for my next post.

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